The present invention relates to refractory liner compositions and, in particular, to refractory liner compositions that will resist the build up of alumina or other oxides during the continuous casting of molten steel.
In the continuous casting of steel, special refractories are used to control the flow of the molten steel and to protect the molten steel from oxidation as it is poured from steel ladles to tundishes and from tundishes to continuous casting moulds. Such refractories include slide gate plates or stopper rods used in molten metal flow control, various collector nozzles in ladles and tundishes, and protective ladle shrouds and submerged pouring nozzles to protect the molten metals from oxidation. These types of speciality refractories are subjected to severe thermal shock, molten steel erosion, and slag attack.
These speciality refractories are usually carbon-containing refractories and more specifically carbon-bonded refractories. They are usually composed of refractory grain such as aluminium oxide, zirconium oxide, clays, magnesium oxide, silicon carbide, silica, or other dense grain of specific mesh size; carbon from flake graphite, amorphous graphite, carbon black, coke, etc.; and a carbonaceous binder derived from pitch or resin.
Some oxidation takes place during the manufacture of steel, and considerable amounts of oxygen may dissolve in the molten metal. In the ensuing solidification of the steel during casting much of the dissolved gas is expelled and, in the case of oxygen, reacts with carbon to produce evolved carbon monoxide. The dispelled oxygen, carbon monoxide and other gases create undesirable porosity, cracks, and internal defects which lower the quality of the finished steel. In order to eliminate the problem of dissolved oxygen, molten steels are deoxidized or "killed" by the addition of aluminium metal, ferromanganese, or ferrosilicon. In the case of aluminum-killed steel, the aluminium reacts with the dissolved oxygen or iron oxide to form finely-dispersed aluminium oxide, some of which floats into the slag above the molten steel and some of which remains as highly-dispersed micro particles in the solidified steel. During continuous casting, this extremely fine alumina has a tendency either to precipitate out of the molten steel onto the cooler refractory surfaces or to react and stick to the ceramic refractories that line the molten steel path from ladle to tundish to casting mould.
This precipitated alumina has a particular affinity to the typical carbon-bonded alumina-graphite refractories utilized as ladle shrouds and submerged pouring nozzles, which are often referred to as subentry nozzles (SEN). The alumina will continue to build up in the subentry nozzles until the flow of molten steel is reduced to the point that the tube must be lanced by an oxygen torch, or discarded. If oxygen lancing becomes necessary, the casting process is disrupted costing time and money, the casting efficiency decreases, and the quality of the steel must be downgraded. A total alumina blockage of a subentry nozzle decreases the expected life of the refractories and is very costly to steel producers. In aluminum-killed steels where high dissolved-oxygen concentrations are expected, the useful life of a submerged pouring nozle may be limited to 2-3 ladles due to the heavy alumina build up on the interior diameter of the tube.
We have now developed particular refractory compositions that can be formed into interior liner materials on submerged pouring nozzles, ladle shrouds, collector nozzles, etc., and thereby prevent or inhibit the build up of alumina or other oxides thereon during the production of molten steel during continuous casting.